Passage III

Birds have existed since at least 120 million years ago (mya), when Jeholornis was alive. Jeholornis, one of the earliest birds known from fossils, had reptilian characteristics (including teeth, a long bony tail, and claws) and bird Iike characteristics (including feathers, 3 digits on each foot, hollow bones, and a beak ). Two scientists discuss the origin of the first birds.

Scientist 1

The first birds appeared 150 mya. They evolved from a group of ground-dwelling, 3-toed dinosaurs known as theropods. Like the more recent Jeholornis, theropods had long bony tails, teeth, and claws. Theropods had 23 skeletal features that were identical to the corresponding skeletal features of the first birds.

Although theropods could not fly, a 225-million-year old fossil of a feather-covered theropod has shoulder joints that show the theropod could fold its forelimbs close to its body the way a bird folds its wings. The first birds had a nearly identical shoulder joint and could fly. Fossils of other feather-covered, nonflying theropods that predated Jeholornis have also been discovered.

Scientist 2

The first birds appeared 220 mya. They evolved from a group of tree-dwelling reptiles known as archosaurs instead of from dinosaurs. Some archosaurs could glide to the ground; others could also fly. The 230-million-year-old fossil of an archosaur indicates that the archosaur had numerous elongated scales along its front limbs. Those scales had some characteristics identical to those of feathers. This archosaur could have glided from trees to the ground, as could Jeholornis. Theropods could not be the ancestors of birds, because theropods lived only on the ground.

The fact that some skeletal features of theropods were identical to those of the first birds is not evidence that theropods were the most recent ancestors of birds. Instead, these features are independent adaptations to similar habitats. A study matched each of the 3 digits on the feet of theropods and on the feet of the first birds to one of the 5 digits on the feet of various earlier reptiles. Theropods had the 1st, 2nd, and 3rd digits of the original 5 digits, whereas the first birds had the 2nd, 3rd, and 4th digits.